I would just save yourself (herself) some trouble and see if you can trade it in for a new one.
Most of the time the fish looking like this was from illness at the store. Rough shipping, ammonia often and she's just got a poor fish.
What are her tank parameters.
How long has the tank been set up and running and how long have the other fish been in the tank?
In my signature is an article with links for a couple others, with fish life saving information about fish keeping.
Usually a newer tank, isn't actually cycled. The fish stores say set up your tank come back in a couple days and your tank will be ready for fish.
But a cycled tank, needs a colony of BB / beneficial bacteria to process fish waste and uneaten food which turns into ammonia which is toxic to fish. That usually takes months to establish, or you use someones filter pad to seed the new pad or sometimes good quality bacteria in a bottle can help speed up the time it takes.
If it is fin rot the best 1st thing is to simply have perfect water parameters. Daily significant water changes will be very useful. Being that the fish isn't dealing with surviving unsafe water parameters, and it's body can use that energy to heal up.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves